i had those marks from my side guards, i used a tool from the body shop to get most of it off, then i used some of that 3m industrial strenth adhesive remover, soaked the area, and scratched it off with my nails...
diesel fuel will take that off and wont damage paint. i use diesel fuel to remove road tar in the underside of the doors and stuff. and it seems to leave a watter reppeling coating on the car too after its used.